Light emitting device

ABSTRACT

A light emitting device includes a carrier, a plurality of light emitting diode chips and a plurality of buffer pads. Each light emitting diode chip includes a first type semiconductor layer, an active layer, a second type semiconductor layer, a via hole and a plurality of bonding pads. The via hole sequentially penetrates through the first type semiconductor layer, the active layer and a portion of the second type semiconductor layer. The first type semiconductor layer, the active layer, the second type semiconductor layer and the via hole define a epitaxial structure. The buffer pads are disposed between the carrier and the second type semiconductor layer, wherein the buffer pads is with Young&#39;s modulus of 2˜10 GPa, the second bonding pad is disposed within the via hole to contact the second type semiconductor layer, and the epitaxial structure is electrically bonded to the receiving substrate through the bonding pads.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application of and claims the priority benefit of U.S. application Ser. No. 15/001,250, filed on Jan. 20, 2016, now pending, which claims the priority benefit of Taiwan application serial no. 104129262, filed on Sep. 4, 2015. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a semiconductor device, and particularly relates to a light emitting device.

Description of Related Art

In general, a light emitting chip is composed of an epitaxial structure, an N type electrode, and a P type electrode. In addition, the N type electrode and the P type electrode may respectively contact an N type semiconductor layer and a P type semiconductor layer. To make the light emitting chip more applicable, the light emitting chip that is manufactured may be heated for metal bonding to be fixed to a circuit board, so as to form a light emitting module. Due to a mismatch between thermal expansion coefficients of materials of the light emitting chip and the circuit board, a thermal stress and an internal stress that are generated become more and more significant. Thus, there may be dislocation between the epitaxial structure of the light emitting chip and the circuit board, and a structural reliability of the product consequently becomes less desirable.

SUMMARY OF THE INVENTION

The invention provides a light emitting device suitable to be disposed to various receiving substrates and having a preferable structural reliability.

A light emitting device according to an embodiment of the invention includes a carrier, a plurality of light emitting diode chips and a plurality of buffer pads. The light emitting diode chips are disposed on the carrier, wherein each light emitting diode chip includes a first type semiconductor layer, an active layer, a second type semiconductor layer, a via hole and a plurality of bonding pads. The active layer is disposed on the first type semiconductor layer. The second type semiconductor layer is disposed on the active layer. The via hole sequentially penetrates through the first type semiconductor layer, the active layer and a portion of the second type semiconductor layer. The first type semiconductor layer, the active layer, the second type semiconductor layer and the via hole define an epitaxial structure. The bonding pads are disposed between the epitaxial structure and a receiving substrate and includes a first bonding pad electrically connected to the first type semiconductor layer and a second bonding pad electrically connected to the second type semiconductor layer. The buffer pads are located on the carrier and disposed between the carrier and the second type semiconductor layer of the epitaxial structures, wherein the buffer pads are with Young's modulus of 2˜10 GPa, the second bonding pad is disposed within the via hole to contact the second type semiconductor layer, and the epitaxial structures are electrically bonded to the receiving substrate through the bonding pads.

According to an embodiment of the invention, a material of the buffer pads is organic with Young's modulus of 2.9˜3.6 GPa.

According to an embodiment of the invention, the material of the buffer pads is organic with break elongation 5-10% and residual stress 30˜40 MPa.

According to an embodiment of the invention, the receiving substrate comprises a plurality of circuit electrodes, and the bonding pads are electrically connected to the circuit electrodes.

According to an embodiment of the invention, the each light emitting diode chip further includes an insulating layer. The insulating layer is disposed on the first type semiconductor layer and a sidewall of the via hole, wherein the insulating layer fauns a contact opening to expose the first type semiconductor layer and form another opening in the via hole to expose the second type semiconductor layer and the first bonding pad is disposed in the contact opening, the second bonding pad is disposed in another opening, and a portion of the insulating layer is located between a side wall of the via hole and the second bonding pad.

According to an embodiment of the invention, the receiving substrate comprises a plurality of sub-pixels, each of the epitaxial structures corresponds to one of the sub-pixels.

According to an embodiment of the invention, the receiving substrate comprises a plurality of light-obstructed structures, and the light-obstructed structures and the light emitting diode chips are arranged alternately.

According to an embodiment of the invention, a height of each light-obstructed structure is higher than a position of the active layer of the adjacent epitaxial structure.

According to an embodiment of the invention, the thickness of the second type semiconductor layer is 3 times to 15 times of a thickness of the active layer, and the thickness of the second type semiconductor layer is 10 times to 20 times of the thickness of the first type semiconductor layer.

According to an embodiment of the invention, an area of an orthogonal projection of each of the buffer pads on the carrier is 0.6 times to 1.2 times of an area of an orthogonal projection of the epitaxial structure on the carrier.

According to an embodiment of the invention, in a vertical direction with respect to the carrier, each of the buffer pads and the epitaxial structure are conformal patterns.

According to an embodiment of the invention, the receiving substrate comprises a thin film transistor substrate, a complementary metal oxide semiconductor substrate, a printed circuit board substrate or a flexible substrate.

According to an embodiment of the invention, an orthogonal projection area of the first bonding pad on the receiving substrate is the same as an orthogonal projection area of the second bonding pad on the receiving substrate.

According to an embodiment of the invention, an orthogonal projection area of the first bonding pad on the receiving substrate is different from an orthogonal projection area of the second bonding pad on the receiving substrate.

A light emitting device according to an embodiment of the invention includes a carrier, a plurality of light emitting diode chips and a plurality of buffer pads. The light emitting diode chips are disposed on the carrier, wherein each light emitting diode chip includes a first type semiconductor layer, an active layer, a second type semiconductor layer, a via hole and a plurality of bonding pads. The active layer is disposed on the first type semiconductor layer. The second type semiconductor layer is disposed on the active layer. The via hole sequentially penetrates through the first type semiconductor layer, the active layer and a portion of the second type semiconductor layer, wherein the first type semiconductor layer, the active layer, the second type semiconductor layer and the via hole define an epitaxial structure. The bonding pads are disposed between the epitaxial structure and a receiving substrate and includes a first bonding pad electrically connected to the first type semiconductor layer and a second bonding pad electrically connected to the second type semiconductor layer. The buffer pads are located on the carrier and disposed between the carrier and the second type semiconductor layers of the epitaxial structures, wherein the buffer pads are with Young's modulus of 2˜10 GPa, the second bonding pad is disposed within the via hole to contact the second type semiconductor layer, the epitaxial structures are electrically bonded to the receiving substrate through the bonding pads, and a gap between any adjacent two of the buffer pads is 0.2 times to 2 times of a side length of each of the epitaxial structures.

According to an embodiment of the invention, a material of the buffer pads is organic with Young's modulus of 2.9˜3.6 GPa.

According to an embodiment of the invention, the material of the buffer pads is organic with break elongation 5˜10% and residual stress 30˜40 MPa.

According to an embodiment of the invention, the receiving substrate comprises a plurality of light-obstructed structures and the light-obstructed structures and the light emitting diode chips are arranged alternately.

According to an embodiment of the invention, a height of each light-obstructed structure is higher than a position of the active layer of the adjacent epitaxial structure.

Based on above, since the light emitting device according to the embodiments of the invention has the buffer pad, the buffer pad may absorb the internal stress generated when the light emitting device is disposed on the receiving substrate and the bonding pad is thermally bonded to the receiving substrate, and the movement generated when a high stress is applied to the epitaxial structure is also reduced. In brief, the buffer pad may prevent the dislocation between the epitaxial structure and the receiving substrate. Thus, the structural design of the light emitting device according to the embodiments of the invention helps the subsequent thermal bonding process, and may effectively improve the structural reliability of the light emitting device.

In order to make the aforementioned and other features and advantages of the invention comprehensible, several exemplary embodiments accompanied with figures are described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 A is a schematic cross-sectional view illustrating a light emitting device according to an embodiment of the invention.

FIG. 1B is a schematic cross-sectional view illustrating a light emitting device according to another embodiment of the invention.

FIG. 2 is a schematic cross-sectional view illustrating a light emitting device according to another embodiment of the invention.

FIG. 3 is a schematic cross-sectional view illustrating a light emitting device according to another embodiment of the invention.

FIG. 4 is a schematic cross-sectional view illustrating the light emitting device of FIG. 3 being thermally bonded to a receiving substrate.

FIG. 5 is a schematic cross-sectional view illustrating a light emitting device according to another embodiment of the invention.

FIG. 6 is a schematic cross-sectional view illustrating the light emitting device of FIG. 5 being thermally bonded to a receiving substrate.

FIG. 7A is a schematic cross-sectional view illustrating a light emitting device according to another embodiment of the invention.

FIG. 7B is a schematic cross-sectional view illustrating a light emitting device according to another embodiment of the invention.

FIG. 8A is a schematic top view illustrating a display panel according to an embodiment of invention.

FIG. 8B is a schematic partial cross-sectional view of the display panel depicted in FIG. 8A.

FIG. 9 is a schematic cross-sectional view illustrating a light emitting device according to another embodiment of the invention.

FIG. 10 is a schematic cross-sectional view illustrating a display panel according to another embodiment of the invention.

FIG. 11 is a schematic cross-sectional view illustrating a display panel according to another embodiment of the invention.

FIG. 12 is a schematic cross-sectional view illustrating a display panel according to another embodiment of the invention.

FIG. 13 is a schematic cross-sectional view illustrating a display panel according to another embodiment of the invention.

FIG. 14A is a schematic cross-sectional view illustrating a display panel according to another embodiment of the invention.

FIG. 14B is a schematic cross-sectional view illustrating a display panel according to another embodiment of the invention.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1A is a schematic cross-sectional view illustrating a light emitting device according to an embodiment of the invention. Referring to FIG. 1A, in this embodiment, a light emitting device 100 a includes a carrier 110, at least one epitaxial structure 120 (FIG. 1A only shows one epitaxial structure), at least one buffer pad 130 (FIG. 1A only shows one buffer pad 130), and at least one bonding pad 140 a (FIG. 1A only shows one bonding pad 140 a). The epitaxial structure 120 is disposed on the carrier 110. The buffer pad 130 is disposed between the carrier 110 and the epitaxial structure 120, wherein the epitaxial structure 120 is temporarily bonded to the carrier 110 by the buffer pad 130. The bonding pad 140 a is disposed on the epitaxial structure 120, wherein the epitaxial structure 120 is electrically connected to a receiving substrate (not shown) through the bonding pad 140 a.

Specifically, the carrier 110 of this embodiment is substantially a tentative substrate for temporarily carrying the epitaxial structure 120. In addition, a material of the carrier 110 may include Si, SiC, GaAs, GaN, glass, sapphire, or ceramics, for example. The epitaxial structure 120 includes a first type semiconductor layer 122, an active layer 124, and a second type semiconductor layer 126. In addition, the active layer 124 is located between the first type semiconductor layer 122 and the second type semiconductor layer 126, and the second type semiconductor layer 126 is located between the active layer 124 and the buffer pad 130. In the epitaxial structure 120 of this embodiment, the first type semiconductor layer 122 is a P type semiconductor layer, for example, the second type semiconductor layer 126 is an N type semiconductor layer, for example, and the active layer 124 is a multiple quantum well (MQW) structure. In other embodiments not shown herein, it is also viable that the first type semiconductor layer 122 is an N type semiconductor layer, for example, the second type semiconductor layer 126 is a P type semiconductor layer, for example, and the active layer 124 is a multiple quantum well structure. The invention does not intend to impose a limitation in this regard.

More specifically, the epitaxial structure 120 may include a Group II-VI material (e.g., ZnSe) or a Group III-V nitride material (e.g., GaN, MN, InN, InGaN, AlGaN, or AlInGaN). In addition, a side length of the epitaxial structure 120 of the embodiment is smaller than the side length of the conventional light emitting diode (e.g., the side length in a range from 0.2 millimeters to 1 millimeter). Preferably, the side length of the epitaxial structure 120 is in a range from 3 micrometers to 40 micrometers. In this embodiment, a bottom surface 120 b of the epitaxial structure 120 is wider than a top surface 120a of the epitaxial structure 120, and a cross-sectional side of the epitaxial structure may be deemed as a trapezoid. More specifically, a side length of the first type semiconductor layer 122 is smaller than a side length of the second type semiconductor layer 126, and a difference in side lengths between the first type semiconductor layer 122 and the second type semiconductor layer 126 is in a range from 0.5 micrometers to 5 micrometers. Preferably, the side length of the first type semiconductor layer 122 is in a range from 3 micrometers to 38 micrometers, and the side length of the second type semiconductor layer 126 is in a range from 3.5 micrometers to 40 micrometers. In other embodiments not shown herein, the top surface 120 a of the epitaxial structure 120 may also be wider than the bottom surface 120 b of the epitaxial structure 120. The cross-sectional side of the epitaxial structure 120 may be deemed as an inverted trapezoid. Namely, the side length of the first type semiconductor layer 122 is greater than the side length of the second type semiconductor layer 126, and the difference in side lengths between the first type semiconductor layer 122 and the second type semiconductor layer 126 is in a range from 0.5 micrometers to 5 micrometers. Preferably, the side length of the first type semiconductor layer 122 is in a range from 3.5 micrometers to 40 micrometers, and the side length of the second type semiconductor layer 126 is in a range from 3 micrometers to 38 micrometers. Furthermore, in yet other embodiments not shown herein, the bottom surface 120 b of the epitaxial structure 120 and the top surface 120 a of the epitaxial structure 120 may have similar side lengths, and the cross-sectional side of the epitaxial structure 120 may be deemed as a rectangle. Namely, the side length of the first type semiconductor layer 122 and the side length of the second type semiconductor layer 126 are equal. Preferably, the side lengths of the first type semiconductor layer 122 and the second type semiconductor layer 126 are respectively in a range from 3 micrometers to 40 micrometers. Among the three types of embodiments, the design that the bottom surface 120 b of the epitaxial structure 120 is wider than the top surface 120 a of the epitaxial structure 120 (i.e., the side length of the first type semiconductor layer 122 is smaller than the side length of the second type semiconductor layer 126 and the cross-sectional side of the epitaxial structure 120 is deemed as a trapezoid) is advantageous in having a greater contact area between the epitaxial structure 120 and the buffer pad 130, thus having a desirable stress releasing effect.

As shown in FIG. 1A, in the epitaxial structure 120 of this embodiment, the second type semiconductor layer 126 is close to the buffer pad 130, and the second type semiconductor layer 126 directly contacts the buffer pad 130. Moreover, a thickness of the second type semiconductor layer 126 of this embodiment is greater than a thickness of the first type semiconductor layer 122. In addition, the thickness of the second type semiconductor layer 126 is in a range from 1 micrometer to 6 micrometers, a thickness of the active layer 124 is in a range from 0.1 micrometers to 1 micrometer, and the thickness of the first type semiconductor layer 122 is in a range from 0.1 micrometers to 0.5 micrometers. In an exemplary embodiment, the thickness of the second type semiconductor layer 126 is 5 micrometers, for example, the thickness of the active layer 124 is 0.7 micrometers, for example, and the thickness of the first type semiconductor layer. 122 is 0.4 micrometers, for example. Moreover, the second type semiconductor layer 126 in the epitaxial structure 120 has the greatest thickness. The thickness of the second type semiconductor layer 126 is 3 times to 15 times of the thickness of the active layer 124, and the thickness of the second type semiconductor layer 126 is 10 times to 20 times of the thickness of the first type semiconductor layer 122. By having the thickest second type semiconductor layer 126 directly contacting the buffer pad 130, the active layer 124 may be protected to prevent the structure of the active layer 124 from being damaged to thus influence light emitting of the light emitting device 100 a.

Moreover, a highest peak current density of an external quantum efficiency curve of the epitaxial structure 120 of this embodiment is smaller than the peak current density of the epitaxial structure of the conventional light emitting diode. Preferably, the highest peak current density of the external quantum efficiency curve of the epitaxial structure 120 of this embodiment is lower than 2 A/cm². More preferably, the highest peak current density of the external quantum efficiency curve of the epitaxial structure 120 of this embodiment is in a range from 0.5 A/cm² to 1.5 A/cm². Namely, the epitaxial structure 120 of this embodiment is suitable to be operated at a low current density. In addition, compared to the epitaxial structure of the conventional light emitting diode, a defect density of the epitaxial structure 120 of this embodiment is also smaller. In general, the defect density of the epitaxial structure of the conventional light emitting diode is approximately in a range from 10⁹/cm² to 10¹⁰/cm², while the defect density of the epitaxial structure 120 of this embodiment is smaller than 5×10⁸/cm², and preferably in a range from 5×10⁵/cm² to 10⁸/cm². Moreover, the buffer pad 130 of this embodiment may serve as a buffer structure. A material of the buffer pad 130 of this embodiment is an organic, for example, an adhesive polymer, such as epoxy resin, polyimide, polyester, polyurethane, benzocyclobutene, polyethylene, polypropylene, polyacrylate, and a combination thereof, and the buffer pad 130 may be formed through thermal curing or UV curing. In other words, the buffer pad 130 may simultaneously offer adhesive and buffering functions. In an exemplary embodiment, the buffer pad 130 is formed of a polymer through UV curing. In an exemplary embodiment, a material of the buffer pad 130 is with Young's Modules of 2.9˜3.6 GPa, break elongation of 5˜10%, or residual stress of 30˜40 MPa.

As shown in FIG. 1A, a position of the buffer pad 130 and a position of the epitaxial structure 120 in this embodiment correspond to each other. In addition, an area of an orthogonal projection of the buffer pad 130 on the carrier 110 is 0.6 times to 1.2 times of an area of an orthogonal projection of the epitaxial structure 120 on the carrier 110. The buffer pad 130 is structurally designed to ensure that a stress applied to any position of the epitaxial structure 120 may be absorbed by the buffer pad 130. The buffer pad 130 may be a single-layer or multi-layer structure. For example, the buffer pad 130 may be a dual-layer structure formed of two polymer materials or a multi-layer structure formed by alternately stacking two polymer materials. However, the invention is not limited thereto. Besides, as shown in FIGS. 1A and 1B, the epitaxial structures 120 and 120′ may serve as a mask and an etching process may be performed to define shapes and sizes of the buffer pads 130 and 130′. In other words, viewing from a vertical direction H with respect to the carrier 110, the buffer pads 130 and 130′ and the epitaxial structures 120 and 120′ of the light emitting devices 100 a and 100 a′ of the embodiments are shown as substantially similar patterns. As shown in FIG. 1A, the cross-sectional side of the epitaxial structure 120 that serves as a mask is deemed as a trapezoid. Therefore, the size of the buffer pad 130 defined after the etching process is slightly greater than that of the epitaxial structure 120. As shown in FIG. 1B, the cross-sectional side of the epitaxial structure 120′ serving as a mask is deemed as a rectangle. Therefore, the buffer pad 130′ defined after the etching process is similar to the epitaxial structure 120′. In other embodiments not shown herein, when the cross-sectional side of the epitaxial structure 120 is deemed as an inverted trapezoid, the size of the buffer pad 130 defined after the etching process is slightly smaller than the epitaxial structure 120. More specifically, a relation between the size of the buffer pad 130 and the size of the epitaxial structure 120 may be controlled by adjusting parameters of the etching process.

Besides, the bonding pad 140 a of this embodiment is located on the first type semiconductor layer 122 and is structurally and electrically connected to the first type semiconductor layer 122. The epitaxial structure 120 of this embodiment may be electrically connected to the receiving substrate (not shown) through the bonding pad 140 a, so as to improve an applicability of the light emitting device 100 a. The bonding pad 140 a may include one or multiple layers. For example, the bonding pad 140 a may include an electrode layer (not shown) and an optionally disposed barrier layer (not shown). The electrode layer may form an ohmic contact with the first type semiconductor layer 122, and a material of the electrode layer may include a high work function metal (e.g., platinum, nickel, titanium, gold, chromium, and a combination thereof) or metal oxide (e.g., indium tin oxide and zinc oxide). The barrier layer may be optionally disposed to prevent impurities from diffusing into the first type semiconductor layer 122. For example, a material of the barrier layer includes, but is not limited to titanium-tungsten alloy, platinum, palladium, titanium, tantalum, and a combination thereof. The bonding pad 140 a may further include a reflective layer (not shown) to reflect light emitted by the active layer 124. For example, the reflective layer includes, but is not limited to, silver, aluminum, and an alloy thereof. Furthermore, the bonding pad 140 a may also include a non-metal conductive material, such as conductive polymer, graphite, graphene, and black phosphorus. In this embodiment, the bonding pad 140 a may be formed of a metal material, a non-metal material, or a combination of metal and non-metal materials, as long as the bonding pad 140 a makes the epitaxial structure 120 and the receiving substrate (not shown) electrically connect to each other. When being electrically connected to the receiving substrate, the bonding pad 140 a of the light emitting device 100 a is aligned and pressed to a pad (not shown) of the receiving substrate through hot pressing. Under a high temperature, a high stress is applied for a period of time for bonding. At this time, the buffer pad 130 of the light emitting device 100 a may serve as a buffer to absorb an internal stress generated during bonding, so as to reduce a movement generated when the high stress is applied to the epitaxial structure 120, thereby preventing dislocation between the epitaxial structure 120 and the receiving substrate.

It should be noted that the reference numerals and a part of the contents in the previous embodiment are used in the following embodiments, in which identical reference numerals indicate identical or similar components, and repeated description of the same technical contents is omitted. For a detailed description of the omitted parts, reference can be found in the previous embodiment, and no repeated description is contained in the following embodiments.

FIG. 2 is a schematic cross-sectional view illustrating a light emitting device according to another embodiment of the invention. Referring to FIGS. 1A and 2 together, a light emitting device 100b of this embodiment is similar to the light emitting device 100 a shown in FIG. 1A, except for a difference that the light emitting device 100 b of this embodiment further includes an insulating layer 150. The insulating layer 150 is disposed on the buffer pad 130 and covers a sidewall of the epitaxial structure 120, and the insulating layer 150 exposes the top surface 120 a of the epitaxial structure 120 to form a contact opening O. A width of the contact opening O is smaller than a width of the top surface 120 a of the epitaxial structure 120, and the bonding pad 140 a is disposed on the contact opening O and electrically connected to the epitaxial structure 120. In other embodiments not shown herein, the width of the contact opening O may be greater than, close to, or approximately equal to the width of the top surface 120 a of the epitaxial structure 120, and the invention is not limited thereto. The insulating layer 150 is disposed to protect an edge of the epitaxial structure 120, so as to prevent the epitaxial structure 120 from moisture and oxygen, and may effectively improve a product reliability of the light emitting device 100 b. A material of the insulating layer 150 includes silicon dioxide, aluminum oxide, silicon nitride, and a combination thereof, for example.

FIG. 3 is a schematic cross-sectional view illustrating a light emitting device according to another embodiment of the invention. Referring to FIGS. 1 A and 3 together, a light emitting device 100 c of this embodiment is similar to the light emitting device 100 a shown in FIG. 1A, except for a main difference that the light emitting device 100 c of this embodiment includes a plurality of the epitaxial structures 120 and a plurality of the buffer pads 130. In addition, the epitaxial structures 120 are periodically and separately disposed on the carrier 110, and the buffer pads 130 are respectively disposed in correspondence with the epitaxial structures 120. In an embodiment, the epitaxial structure 120 is in an array arrangement, and a pitch P1 between any adjacent two epitaxial structures 120 is in a range from 2 micrometers to 70 micrometers, and a pitch P2 between any adjacent two buffer pads 130 is in a range from 2 micrometers to 70 micrometers. Regarding subsequent processes, referring to FIG. 4, when the light emitting device 100 c is electrically connected to a receiving substrate 10 through a thermal bonding process, the bonding pad 140 a may be electrically connected to a pad 20 on the receiving substrate 10. At this time, the buffer pad 130 may absorb the internal stress generated during bonding and reduce the movement generated when the high stress is applied to the epitaxial structure 120. In this embodiment, the receiving substrate 10 may be a display substrate, a lighting substrate, a substrate having transistors or integrated circuits (ICs), or a substrate having metal redistribution lines. More specifically, the pad 20 is formed of a material having a melting temperature lower than 140° C., such as tin or indium. During a thermal bonding process with such design, the receiving substrate 10 is heated to a temperature higher than the melting temperature of the pad 20 and lower than a melting temperature of the bonding pad 140 a. At such temperature, the pad 20 is turned into a liquid state, while the bonding pad 140 a remains in a solid state, and when the pad 20 and the bonding pad 140 a are connected, the liquid-state pad 20 may reduce a collision force at a contact surface, so as to prevent the light emitting device 100 c from tilting and assuage imbalance among forces applied to the respective epitaxial structures 120 when bonding.

In this embodiment, the pad 20 is aligned to (co-planer to) the receiving substrate 10. However, the invention is not limited thereto. In other embodiments not shown herein, the pad 20 may also be a block protruding from the receiving substrate 10. Namely, an upper surface of the pad 20 is higher than an upper surface of the receiving substrate 10. Alternatively, the pad 20 may also be a block recessed into the receiving substrate 10. Namely, the upper surface of the pad 20 is lower than the upper surface of the substrate 10. Besides, the upper surface of the receiving substrate 10 between the pads 20 may form a light absorption layer 18 to absorb a portion of scattered light, so as to reduce a mutual light interference between the epitaxial structures 120.

In this embodiment, the pitch P2 between any adjacent two buffer pads 130 is 0.2 times to 2 times of the side length of the epitaxial structure 120. Preferably, the pitch P2 between any adjacent two buffer pads 130 is smaller than the side length of the epitaxial structure 120, and the pitch P2 between any adjacent two buffer pads 130 is 0.2 times to 0.9 times of the side length of the epitaxial structure 120. If the pitch P2 is smaller than 0.2 times of the side length of the epitaxial structure 120, the buffer pad 130 may expand due to a stress and contact the adjacent buffer pad 130 during the thermal bonding process, thus resulting in a movement of the corresponding epitaxial structure 120. If the pitch P2 is greater than 2 times of the side length of the epitaxial structure 120, a buffer area in the thermal bonding process is not enough, and the epitaxial structure 120 may be damaged easily. In brief, the buffer pad 130 may prevent the dislocation between the epitaxial structure 120 and the receiving substrate 10 and protect the epitaxial structure 120 as well. Thus, the structural design of the light emitting device 100 c of this embodiment helps the subsequent thermal bonding process, and may effectively improve a structural reliability of the light emitting device 100 c. In addition, since the thickness of the second type semiconductor layer 126 is greater than the thickness of the first type semiconductor layer 122, i.e., the thickness of the first type semiconductor layer 122 is smaller than the thickness of the second type semiconductor layer 126, when the light emitting device 100 c is bonded to the receiving substrate 10, the active layer 124 of the epitaxial structure 120 may be closer to the substrate 10, thus resulting in a preferable heat dissipation effect. It should be noted that, after the thermal bonding process, the buffer pad 130 and the carrier 110 may be removed, and another bonding pad (not shown) may be manufactured on the second type semiconductor layer 126 of the epitaxial structure 120. At this time, the epitaxial structure 120, the bonding pad 140 a, and the another bonding pad may define a vertically oriented light emitting diode chip.

Besides, the invention does not limit the position and the number of the bonding pad 140 a. Here, the bonding pad 140 a is only disposed on a surface of the epitaxial structure 120 away from the buffer pad 130 and each epitaxial structure 120 is only provided with one bonding pad 140 a, However, in other embodiments, referring to FIGS. 3 and 5 together, a light emitting device 100 d of this embodiment is similar to the light emitting device 100 c shown in FIG. 3, except for a main difference that a bonding pad 140 b of the light emitting device 100 d of this embodiment includes a first bonding pad 142 b and a second bonding pad 144 b. The first bonding pad 142 b and the second bonding pad 144 b are located at the same side of the epitaxial structure 120. In addition, the first bonding pad 142 b is electrically connected to the first type semiconductor layer 122, and the second bonding pad 144 b is electrically connected to the second type semiconductor layer 126. In other words, each epitaxial structure 120 in this embodiment is provided with a first bonding pad 142 b and a second bonding pad 144 b, and the first bonding pad 142 b and the second bonding pad 144 b are located on a surface of the epitaxial structure 120 away from the buffer pad 130. At this time, the design of the epitaxial structure 120, the first bonding pad 142 b, and the second bonding pad 144 b may be considered as a horizontally oriented light emitting diode chip.

Besides, the light emitting device 100 d may further include the insulating layer 150. The insulating layer 150 is disposed on the buffer pad 130 and covers the epitaxial structure 120, and the insulating layer 150 exposes the first bonding pad 142 b and the second bonding pad 144 b. The insulating layer 150 is disposed to protect the edge of the epitaxial structure 120, so as to prevent the epitaxial structure 120 from moisture and oxygen, and may effectively improve a product reliability of the light emitting device 100 d. The material of the insulating layer 150 includes silicon dioxide, aluminum oxide, silicon nitride, and a combination thereof, for example. In the subsequent process, referring to. FIG. 6, when the light emitting device 100 d is electrically connected to the receiving substrate 10 through the thermal bonding process, the first bonding pad 142 b and the second bonding pad 144 b may be electrically connected to the pad 20 on the receiving substrate 10. For example, the receiving substrate may be a display substrate, a lighting substrate, a substrate having transistors or integrated circuits, or a substrate having metal redistribution lines. At this time, the buffer pad 130 may absorb the internal stress generated during bonding and reduce the movement generated when the high stress is applied to the epitaxial structure 120. In brief, the buffer pad 130 may prevent the dislocation between the epitaxial structure 120 and the receiving substrate 10. Thus, the structural design of the light emitting device 100 d of this embodiment helps the subsequent thermal bonding process, and may effectively improve a structural reliability of the light emitting device 100 d.

FIG. 7A is a schematic cross-sectional view illustrating a light emitting device according to another embodiment of the invention. Referring to FIG. 7A, in this embodiment, a light emitting device 100 e includes the carrier 110, a plurality of epitaxial structures 120 e, the buffer pads 130, and a plurality of bonding pads 140 e. The epitaxial structures 120 e are periodically and separately disposed on the carrier 110, and the buffer pads 130 are respectively disposed in correspondence with the epitaxial structures 120 e. Each of the epitaxial structures 120 e includes a first type semiconductor layer 122 e, an active layer 124 e, a second type semiconductor layer 126 e and a via hole 128 e. The active layer 124 e is disposed on the first type semiconductor layer 122 e. The second type semiconductor layer 126 e is disposed on the active layer 124 e. The via hole 128 e sequentially penetrates through the first type semiconductor layer 122 e, the active layer 124 e and a portion of the second type semiconductor layer 126 e.

Further, the buffer pads 130 of this embodiment are disposed between the carrier 110 and the second type semiconductor layers 126 e of the epitaxial structures 120 e, wherein the epitaxial structures 120 e are connected to the carrier 110 by the buffer pads 130, respectively. The bonding pads 140 e are disposed on the epitaxial structures 120 e and includes at least one first bonding pad 142 e (FIG. 7A shows a plurality of first bonding pad) and at least one second bonding pad 144 e (FIG. 7A shows a plurality of second bonding pad). Each epitaxial structure 120 e in this embodiment is provided with one first bonding pad 142 e and one second bonding pad 144 e, and the first bonding pad 142 e and the second bonding pad 144 e are located at the same side of the epitaxial structure 120 e and on a surface 123 e of the first type semiconductor layer 122 e away from the corresponding buffer pad 130. Specifically, the first bonding pad 142 e is electrically connected to the first type semiconductor layer 122 e. The second bonding pad 144 e is disposed within the via hole 128 e from the surface 123 e of the first type semiconductor layer 122 e relatively far away from the carrier 110 so as to electrically connect to the second type semiconductor layer 126 e. The epitaxial structure 120 e is electrically connected to a receiving substrate 10 e through the bonding pads 140 e. Herein, the receiving substrate 10 e, for example, is a thin film transistor substrate, a complementary metal oxide semiconductor substrate, a printed circuit board substrate or a flexible substrate, and the invention is not limited thereto.

Besides, as shown in FIG. 7A, the light emitting device 100 e further includes at least one insulating layer 150 e (FIG. 7A shows a plurality of insulating layer). In each of the epitaxial structures 120 e, the insulating layer 150 e is disposed on the first type semiconductor layer 122 e and a sidewall of the via hole 128 e, wherein the insulating layer 150 e includes a contact opening O′ to expose the surface 123 e of the first type semiconductor layer 122 e and another opening O″ in the via hole 128 e to expose the second type semiconductor layer 126 e. The first bonding pad 142 e is disposed on the insulating layer 150 e and electrically contacts the first type semiconductor layer 122 e through the contact opening O′. The second bonding pad 144 e is also dispose on the insulating layer 150 e and electrically contacts the second type semiconductor layer 126 e through the opening O″. In other words, the insulating layer 150 e is located between the side wall of the via hole 128 e and the second bonding pad 144 e, and is disposed to electrically isolate the second bonding pad 144 e from the first type semiconductor layer 122 e and the active layer 124 e.

FIG. 7B is a schematic cross-sectional view illustrating a light emitting device according to another embodiment of the invention. Referring to FIGS. 7A and 7B together, a light emitting device 100 f of this embodiment is similar to the light emitting device 100 e shown in FIG. 7A, except for a main difference that the receiving substrate 10 f includes a plurality of circuit electrodes 12 f, and the first bonding pads 142 e and the second bonding pads 144 e are electrically connected to the circuit electrodes 12 f. Herein, the receiving substrate 10 f is embodied as a TFT substrate, and the circuit electrode 12 f are electrically connected to the TFT structures 16 f.

Since the light emitting devices 100 e, 100 f have the buffer pads 130, the buffer pads 130 may absorb the stress generated by thermal bonding process. Furthermore, the bonding shift between the light emitting devices 120 e and the bonding pads 140 e will be reduced with the lower stress. In brief, the buffer pads 130 may prevent the dislocation between the epitaxial structures 120 e and the receiving substrates 10 e, 10 f.

After the bonding pads 140 e are thermally bonded to the receiving substrate 10 f, the buffer pads 130 and the carrier 110 will be removed to form a display panel 20. FIG. 8A is a schematic top view illustrating a display panel according to an embodiment of invention. FIG. 8B is a schematic partial cross-sectional view of the display panel depicted in FIG. 8A. Referring to FIGS. 8A and 8B, the receiving substrate 10 f has a display region 11 f and a peripheral region 13 f outside the display region 11 f. Further, the receiving substrate 10 f includes a plurality of sub-pixels 14 f in the display region 11 f, and each of the epitaxial structures 120 e corresponds to one of the sub-pixels 14 f. The epitaxial structures 120 e and the bonding pads 140 e define a plurality of horizontally oriented light emitting diode chips C1, and the light emitting diode chips C1 are periodically and separately disposed on the receiving substrate 10 f. Because of the need for precise alignment and mass transfer, the presence and location of the buffer pads 130 are more important to improve transfer yield of the light emitting diode chips C1 in transferring and bonding process.

FIG. 9 is a schematic cross-sectional view illustrating a light emitting device according to another embodiment of the invention. Referring to FIGS. 7B and 9 together, a light emitting device 100 g of this embodiment is similar to the light emitting device 100 f shown in FIG. 7B, except for a main difference that the receiving substrate 10 g comprises a plurality of light-obstructed structures 18 g, the epitaxial structures 120 e and the bonding pads 140 e define a plurality of light emitting diode chips C1, and the light-obstructed structures 18 g and the light emitting diode chips C1 are arranged alternately. In this embodiment, the light-obstructed structures 18 g are disposed to absorb a portion of scattered light, so as to reduce a light mixing effect between the light emitting diode chips C1 of different color. Preferably, a height of each of the light-obstructed structures 18 g is higher than a position of the active layers 124 e of the epitaxial structures 120 e. In other words, the height of one light-obstructed structure 18 g is larger than a distance between the active layer 124 e of adjacent epitaxial structure 120 e and the receiving substrate 10 g.

FIG. 10 is a schematic cross-sectional view illustrating a display panel according to another embodiment of the invention. Referring to FIGS. 8B and 10 together, a display panel 20 a of this embodiment is similar to the display panel 20 shown in FIG. 8B, except for a main difference that the receiving substrate 10 g has a plurality of light-obstructed structure 19 h, and the light-obstructed structure 19 h and the light emitting diode chips C1 are arranged alternately. Furthermore, the light-obstructed structures 19 h are arranged around one pixel comprising 3 sub-pixels with different color (e.g. R, G, B color) in other embodiment. The light-obstructed structure 19 h are disposed as light barrier to reduce a light mixing effect or improving light extracting performance. Preferably, a height of each of the light-obstructed structures 19 h is higher than a position of the active layers 124 e of the epitaxial structures 120 e. Herein, the light-obstructed structures 19 h may be a reflective layer or a black matrix layer, the invention is not limited thereto. Furthermore, in this embodiment, the display panel 20 a further includes an optical adhesive layer 160 a and a plurality of diffusion particles 162 a dispersed in the optical adhesive layer 160 a. The optical adhesive layer 160 a encapsulates the light emitting diode chips C1. The optical adhesive layer 160 a, the diffusion particles 162 a and the light-obstructed structures 19 h help to increase the light extraction rate, moreover, the optical adhesive layer 160 a can protect the light emitting diode chips C1.

FIG. 11 is a schematic cross-sectional view illustrating a display panel according to another embodiment of the invention. Referring to FIGS. 8B and 11 together, a display panel 20 b of this embodiment is similar to the display panel 20 shown in FIG. 8B, except for a main difference that the display panel 20 b further includes a plurality of optical adhesive layers 160 b and a plurality of diffusion particles 162 b dispersed in the optical adhesive layers 160 b. The optical adhesive layers 160 b at least respectively encapsulate the light emitting diode chips C1 to increase the light extraction efficiency. As shown in FIG. 11, each of the optical adhesive layers 160 b has a substantially lens-shaped structure, so that the light emission effect of the light emitting diode chips C1 can be further improved. Herein, the optical adhesive layer 160 b can be formed by screen printing, dispersing or inkjet printing.

FIG. 12 is a schematic cross-sectional view illustrating a display panel according to another embodiment of the invention. Referring to FIGS. 10 and 12 together, a display panel 20 c of this embodiment is similar to the display panel 20 a shown in FIG. 10, except for a main difference that the first bonding pads 142 c and the second bonding pads 144 c are located at opposite sides of the epitaxial structures 120, respectively, so that the bonding pads 140 c and the epitaxial structures 120 define a plurality of vertically oriented light emitting diode chips C2. The receiving substrate 10 j has a plurality of light-obstructed structures 19 j, and the light-obstructed structures 19 j and the light emitting diode chips C2 are arranged alternately. The light-obstructed structures 19 j are disposed to reduce light mixing between the two adjacent light emitting diode chips C2 of different color. Preferably, a height of each of the light-obstructed structures 19 j is higher than a position of the active layers 124 of the epitaxial structures 120. Furthermore, the receiving substrate 10 j also has a plurality of pixel circuits 17 j, for example, the switch TFT, and the TFT structures 16 j, for example, the driving TFT, are electrically connecting to control the light emitting diode chips. In addition, the display panel 20 c further includes a cover glass 170, a plurality of electrodes 172 located between the cover glass 170 and the light emitting diode chips C2, and a light collecting layer 180 disposed on the cover glass 170 to increase the display brightness. Herein, the light collecting layer 180 in this embodiment is a brightness enhancement film (BEF) or a dual brightness enhancement film (DBEF), the invention is not limited thereto.

FIG. 13 is a schematic cross-sectional view illustrating a display panel according to another embodiment of the invention. Referring to FIGS. 8B and 13 together, a display panel 20 d of this embodiment is similar to the display panel 20 shown in FIG. 8B, except for a main difference that the first bonding pads 142 c and the second bonding pads 144 c are located at opposite sides of the epitaxial structures 120, respectively, so that the bonding pads 140 c and the epitaxial structures 120 define a plurality of vertically oriented light emitting diode chips C2. The vertically oriented light emitting diode chips C2 are periodically and separately disposed on the receiving substrate 10 f, and include at least one red light emitting diode chip C21, at least one blue light emitting diode chip C22 and at least one green light emitting diode chip C23. Herein, the first type semiconductor layers 122 are P type semiconductor layer, and the second type semiconductor layers 126 are N type semiconductor layer. In the red light emitting diode chip C21, the first type semiconductor layers 122 is located between the receiving substrate 10f and the second type semiconductor layers 126. In the blue light emitting diode chip C22 and the green light emitting diode chip C23, the second type semiconductor layers 126 are located between the receiving substrate 10 f and the first type semiconductor layers 122. The above design is because the growth substrate and the epitaxial material of the red light emitting diode chip C21 is different from that of the blue light emitting diode chip C22 and the green light emitting diode chip C23, the transfer process of red light emitting diode chip can be different from the blue light emitting diode chip C22 and the green light emitting diode chip C23. Moreover, in this embodiment, the red light emitting diode chips are p-side down type, the green light emitting diode chips and the blue light emitting diode chips are n-side down type, this structure design could simplify transfer process.

FIG. 14A is a schematic cross-sectional view illustrating a display panel according to another embodiment of the invention. Referring to FIGS. 13 and 14A together, a display panel 20e of this embodiment is similar to the display panel 20 d shown in FIG. 13, except for a main difference that the epitaxial structures 120 e and the bonding pads 140 e define a plurality of horizontally oriented light emitting diode chips C1. The horizontally oriented light emitting diode chips C1 are periodically and separately disposed on the receiving substrate 10 f, and include at least one red light emitting diode chip C11, at least one blue light emitting diode chip C12 and at least one green light emitting diode chip C13. Herein, the first type semiconductor layers 122 e are P type semiconductor layer, and the second type semiconductor layers 126 e are N type semiconductor layer. In the red light emitting diode chip C11, the second type semiconductor layers 128 e is located between the receiving substrate 10 f and the first type semiconductor layers 126 e. In the green light emitting diode chip C12 and the blue light emitting diode chip C13, the first type semiconductor layers 122 e are located between the receiving substrate 10 f and the second type semiconductor layers 126 e. Herein, an orthogonal projection area on the receiving substrate 10 f of each of the first bonding pads 142 e is the same as an orthogonal projection area on the receiving substrate 10 f of each of the second bonding pads 144 e.

FIG. 14B is a schematic cross-sectional view illustrating a display panel according to another embodiment of the invention. Referring to FIGS. 14A and 14B together, a display panel 20 f of this embodiment is similar to the display panel 20 e shown in FIG. 14A, except for a main difference that the size of the first bonding pad142 e′ of the red light emitting diode chip is different from the size of the first bonding pads 142 e′ of the greed/blue light emitting diode chips. The size of the first bonding pad 142 e′ is also different from the size of the second bonding pad 144 e′ of one light emitting diode chip. Specifically, an orthogonal projection area on the receiving substrate 10 f of the first bonding pad 142 e′ of red light emitting diode chip is different from an orthogonal projection area on the receiving substrate 10 f of the first bonding pad 142 e′ of the greed/blue light emitting diode chips.

In view of the foregoing, since the light emitting device according to the embodiments of the invention has the buffer pad, the buffer pad may absorb the internal stress generated during bonding when the light emitting device is subsequently thermally bonded to the receiving substrate and reduce and the movement generated when a high stress is applied to the epitaxial structure. In brief, the buffer pad may prevent the dislocation between the epitaxial structure and the receiving substrate. Thus, the structural design of the light emitting device according to the embodiments of the invention helps the subsequent thermal bonding process, and may effectively improve the structural reliability of the light emitting device.

It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents. 

What is claimed is:
 1. A light emitting device, comprising: a carrier; a plurality of light emitting diode chips, disposed on the carrier, wherein each light emitting diode chip comprises: a first type semiconductor layer; an active layer, disposed on the first type semiconductor layer; a second type semiconductor layer, disposed on the active layer; a via hole, sequentially penetrating through the first type semiconductor layer, the active layer and a portion of the second type semiconductor layer, wherein the first type semiconductor layer, the active layer, the second type semiconductor layer and the via hole define an epitaxial structure; and a plurality of bonding pads, disposed between the epitaxial structure and a receiving substrate and comprising a first bonding pad electrically connected to the first type semiconductor layer and a second bonding pad electrically connected to the second type semiconductor layer; and a plurality of buffer pads, located on the carrier and disposed between the carrier and the second type semiconductor layers of the epitaxial structures, wherein the buffer pads are with Young's modulus of 2˜10 GPa, the second bonding pad is disposed within the via hole to contact the second type semiconductor layer, and the epitaxial structures are electrically bonded to the receiving substrate through the bonding pads.
 2. The light emitting device as claimed in claim 1, wherein a material of the buffer pads is organic with Young's modulus of 2.9˜3.6 GPa.
 3. The light emitting device as claimed in claim 2, wherein the material of the buffer pads is organic with break elongation 5˜10% and residual stress 30˜40 MPa.
 4. The light emitting device as claimed in claim 1, wherein the receiving substrate comprises a plurality of circuit electrodes, and the bonding pads are electrically connected to the circuit electrodes.
 5. The light emitting device as claimed in claim 1, wherein each light emitting diode chip further comprises: an insulating layer, disposed on the first type semiconductor layer and a sidewall of the via hole, wherein the insulating layer forms a contact opening to expose the first type semiconductor layer and forms another opening in the via hole to expose the second type semiconductor layer, and the first bonding pad is disposed in the contact opening, the second bonding pad is disposed in another opening, and a portion of the insulating layer is located between a side wall of the via hole and the second bonding pad.
 6. The light emitting device as claimed in claim 1, wherein the receiving substrate comprises a plurality of sub-pixels, each of the epitaxial structures corresponds to one of the sub-pixels.
 7. The light emitting device as claimed in claim 1, wherein the receiving substrate comprises a plurality of light-obstructed structures, and the light-obstructed structures and the light emitting diode chips are arranged alternately.
 8. The light emitting device as claimed in claim 7, wherein a height of each light-obstructed structure is higher than a position of the active layer of the adjacent epitaxial structure.
 9. The light emitting device as claimed in claim 1, wherein the thickness of the second type semiconductor layer is 3 times to 15 times of a thickness of the active layer, and the thickness of the second type semiconductor layer is 10 times to 20 times of the thickness of the first type semiconductor layer.
 10. The light emitting device as claimed in claim 1, wherein an area of an orthogonal projection of each of the buffer pads on the carrier is 0.6 times to 1.2 times of an area of an orthogonal projection of the epitaxial structure on the carrier.
 11. The light emitting device as claimed in claim 1, wherein in a vertical direction with respect to the carrier, each of the buffer pads and the epitaxial structure are conformal patterns.
 12. The light emitting device as claimed in claim 1, wherein the receiving substrate comprises a thin film transistor substrate, a complementary metal oxide semiconductor substrate, a printed circuit board substrate or a flexible substrate.
 13. The light emitting device as claimed in claim 1, wherein an orthogonal projection area of the first bonding pad on the receiving substrate is the same as an orthogonal projection area of the second bonding pad on the receiving substrate.
 14. The light emitting device as claimed in claim 1, wherein an orthogonal projection area of the first bonding pad on the receiving substrate is different from an orthogonal projection area of the second bonding pad on the receiving substrate.
 15. A light emitting device, comprising: a carrier; a plurality of light emitting diode chips disposed on the carrier, wherein each light emitting diode chip comprises: a first type semiconductor layer; an active layer, disposed on the first type semiconductor layer; a second type semiconductor layer, disposed on the active layer; a via hole, sequentially penetrating through the first type semiconductor layer, the active layer and a portion of the second type semiconductor layer, wherein the first type semiconductor layer, the active layer, the second type semiconductor layer and the via hole define an epitaxial structure; and a plurality of bonding pads, disposed between the epitaxial structure and a receiving substrate and comprising a first bonding pad electrically connected to the first type semiconductor layer and a second bonding pad electrically connected to the second type semiconductor layer; and a plurality of buffer pads, located on the carrier and disposed between the carrier and the second type semiconductor layers of the epitaxial structures, wherein the buffer pads are with Young's modulus of 2˜10 GPa, the second bonding pad is disposed within the via hole to contact the second type semiconductor layer, the epitaxial structures are electrically bonded to the receiving substrate through the bonding pads, and a gap between any adjacent two of the buffer pads is 0.2 times to 2 times of a side length of each of the epitaxial structures.
 16. The light emitting device as claimed in claim 15, wherein a material of the buffer pads is organic with Young's modulus of 2.9˜3.6 GPa.
 17. The light emitting device as claimed in claim 16, wherein the material of the buffer pads is organic with break elongation 5-10% and residual stress 30˜40 MPa.
 18. The light emitting device as claimed in claim 15, wherein the receiving substrate comprises a plurality of light-obstructed structures and the light-obstructed structures and the light emitting diode chips are arranged alternately.
 19. The light emitting device as claimed in claim 18, wherein a height of each light-obstructed structure is higher than a position of the active layer of the adjacent epitaxial structure. 